The present invention relates generally to ink-jet technology and, more specifically, to a pump for circulating ink in an off axis ink delivery system.
The art of inkjet technology is well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines employ inkjet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions.
Basically, it is known in thermal inkjet printing to provide a printhead having an orifice plate in combination with heating elements. Thermal excitation of ink is used to eject droplets through tiny nozzles, or orifices, onto a print media. Dot matrix manipulation of the droplets provides alphanumeric character and graphics printing capabilities. Other ink-jetting mechanisms, such as by the use of piezoelectric transducers or wave propagation as ink droplet generators, are also well developed in the art.
In the most common type of inkjet printer, the printheads are mounted on a scanning carriage which moves back and forth over the print media, printing a swath of text or graphics during each pass over the media. At the end of each sweep over the media, the media is advanced, and the carriage reverses direction. The carriage then scans back across the media in the reverse direction, printing a second swath of text or graphics.
The inkjet pen itself may have a self-contained reservoir for storing ink and providing appropriate amounts of ink to the printhead during a printing cycle. These self-contained pens are also referred to in the art as print cartridges.
If a reusable pen rather than a print cartridge is employed in the hard copy apparatus, ink is generally supplied from a remote, off-axis (or off-board), ink reservoir to a relatively permanent pen body and printhead mechanism. Early inkjet printers used off-axis reservoirs as demonstrated in U.S. Pat. No. 4,312,007 ((Winfield) assigned to the common assignee of the present invention). Moreover, it has been found that for some hard copy applications, such as large format plotting of engineering drawings and the like, there is a requirement for the use of much larger volumes of ink than can be contained within a reasonably sized, replaceable, print cartridge. Therefore, improved, relatively large, off-pen ink reservoir systems have also been developed. Examples are U.S. Pat. No. 4,831,389 (Chan) which shows a multicolor off-board ink supply system, and U.S. Pat. No. 4,929,963 (Balazar) which demonstrates an ink delivery system for an ink-jet printer using a low pressure recirculating pumping system (both assigned to the common assignee of the present invention).
With both print cartridges and off-axis ink supply systems, it is common for the ink supply to be maintained at a slight negative pressure relative to ambient to prevent xe2x80x9cdroolxe2x80x9d of ink from the printheads. It is also common to provide mechanisms to prevent or remove air bubbles and particulate contaminants from the ink, since bubbles and contaminants which find their way into the printheads can cause print quality degradation or, if the air bubbles interfere with cooling of the xe2x80x9cfiringxe2x80x9d resistors, failure of the printhead.
One method commonly used with off-axis ink delivery systems for both insuring the removal of air bubbles and contaminants from the ink and for cooling the printhead is to circulate ink in a path from the supply, through a filter media, through the printhead, and then back to the supply. The continual movement of ink through the printhead prevents the accumulation of air bubbles and the build-up of contaminants in the printhead.
One drawback of ink recirculation systems is that pressure variations in the ink supply or return lines to an inkjet printhead can adversely affect print quality. Current pumps used for ink recirculation, either pressure or suction, either have oscillations in their output induced by the cyclic motion of the pumping components or have a constant pressure but a limited total volume (typical of syringe pumps). Pressure variations will be transmitted to the printhead and result in variations in backpressure, which can result in visual degradation of the printed image.
There is thus a need for pump designs which provide relatively constant pressure during the carriage scan time, and which are compact and simple to manufacture.
Embodiments of the present invention comprise fluid pumps having two expandable chambers, with each chamber having a fluid inlet and a fluid outlet. Each chamber further has a pressurizing wall causing the chamber to expand or contract, thus drawing fluid into the chamber or expelling it from the chamber. The pressurizing walls of the two chambers are in mechanical communication, such that when one chamber is expanding, the other chamber is compressing. Multiple check valves prevent retrograde motion of the ink through the pump.
In an ink recirculation system for a printer having a scanning carriage, embodiments of the present invention take advantage of the turn-around time between print swaths of the carriage to synchronously reverse the driving force on the pump, thus allowing a pump having a relatively small chamber volumes to provide substantially constant pressure during each print swath.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.